Rough electrical contact surface

ABSTRACT

A method for rendering a surface of a contact rough includes submerging the surface of the contact in an electroplating bath having a dissolved metal salt, and pulsing an electric current through the contact and the bath to form a rough metallic structure on the surface of the contact.

This application is a divisional (and claims the benefit of priorityunder 35 USC 120) of U.S. application Ser. No. 08/586,232, filed Jan.12, 1996, now issued as U.S. Pat. No. 5,876,580, Mar. 2, 1999. Thedisclosure of the prior application is considered part of (and isincorporated by reference in) the disclosure of this application.

BACKGROUND

This invention relates to rough electrical contact surfaces. Forexample, rough surfaces may be useful on contact pad surfaces inthin-film membranes used for testing integrated semiconductor devices.The contact pad surfaces make contact with device pads on the surface ofsemiconductor devices. See co-pending U.S. patent application Ser. No.08/303,498. Because device pads can develop oxide layers, the membranecontact pads must penetrate oxide layers or other contaminants to makegood electrical connections.

One method of penetration wears away the oxide layer by force oftencombined with rubbing motion. Another method of penetration uses contactpads having roughened surfaces that can pierce through the oxide layer.The roughened surface can be made, for example, by depositingalready-formed, small, hard (e.g., of rhodium or titanium carbide)particles on the surface of a contact pad, and then electroplating thewhole assembly with a thin layer of nickel.

SUMMARY

In general, in one aspect, the invention features a method for renderinga surface of a contact rough, including submerging the surface of thecontact in an electroplating bath having a dissolved metal salt, andpulsing an electric current through the contact and the bath to form arough metallic structure on the surface of the contact.

Implementations of the invention may feature the following features. Thedissolved metal salt may include nickel chloride. The electroplatingbath may have a concentration of about 150 to about 400 grams of nickelchloride dissolved in a liter of water. The electroplating bath may beheated to about 55 degrees C.±about 5 degrees. The electric current mayhave a current density of between about 35 amps per square foot to about75 amps per square foot. The rough metallic structure may be plated withgold. The electric current may be pulsed for a length of time between 0and about 3 minutes, and may be pulsed with a duty cycle having an onperiod greater than an off period. The ratio of the on period to the offperiod may be between about 4:1 and about 8:1. The on period may bebetween about 0.4 seconds and about 0.8 seconds, and the off period maybe about 0.1 seconds. The ratio of the on period to the off period maybe about 6:1, and the on period may be about 0.6 seconds and the offperiod may be about 0.1 seconds.

The rough metal structure may be comprised of spikes. The spikes may besubstantially conical. The spikes may have a height between about 0.350microns and about 1.275 microns. The spikes may have a base betweenabout 0.345 microns and about 1.250 microns. A side of each of thespikes may have an angle from normal of between about 10 degrees andabout 45 degrees. The spikes may have a density of between about 1 andabout 2 spikes per square micrometer.

In general, in another aspect, the invention provides a roughenedcontact including a contact having a surface, and a solid metal layerdeposited on the surface of the contact and having spikes protrudingaway from the contact.

In general, in another aspect, the invention provides a roughenedcontact including a contact having a surface, a solid metal layerdeposited on the surface of the contact, wherein the solid metal layercomprises nickel, having spikes protruding away from the contact,wherein the spikes are substantially conical, the spikes have a heightbetween about 0.350 microns and about 1.275 microns, the spikes have abase between about 0.345 microns and about 1.250 microns, wherein a sideof each of the spikes has an angle from normal of between about 10degrees and about 45 degrees, and wherein the spikes have a density ofbetween about 1 and about 2 spikes per square micrometer, and aconductive plating deposited on the surface of the solid metal surface,wherein the conductive plating comprises gold.

In general, in another aspect, the invention provides a membrane for usein testing a circuit including a flexible membrane substrate, a contacton the flexible membrane substrate having a surface, and a solid metallayer deposited on the surface of the contact and having spikesprotruding away from the contact.

Implementations of the invention may feature the following features. Thespikes may be substantially conical. The solid metal layer may includenickel. A conductive plating may be deposited on the surface of thesolid metal layer. The conductive plating may include gold. The flexiblemembrane substrate may include a polyimide film.

The advantages of the invention may include one or more of thefollowing. Relatively sharp spikes may be created on the surface of acontact. These spikes may be deposited with a relatively uniform height,and dispersed relatively evenly across the contact surface. These spikesdo not require the dropping of hardened particles onto the contactsurface. Instead, these spikes may be created through a simple,continuous electroplating process. The hard, sharp spikes reduce damageto small and delicate components and form consistently reliableconnections.

Other features and advantages of the invention will become apparent fromthe following description and from the claims.

DESCRIPTION

FIGS. 1a through 1 g are cross-sections of a substrate during a spikedeposition procedure.

FIG. 2 is a schematic diagram of spike deposition apparatus.

FIG. 3 is a timing diagram of applied current during spike deposition.

FIG. 4 is a cross-section of a substrate including particle-less spikes.

FIG. 5 is a cross-section of a single spike.

FIGS. 6a and 6 b are photomicrographs of a contact pad includingparticle-less spikes.

FIGS. 7a through 7 f are contour plots of spike deposition parameters.

FIGS. 8a and 8 b are elevated and cross-section views of a membranehaving a contact pad with spikes.

Referring to FIGS. 1a through 1 g, a particle-less spike depositionprocedure begins with a substrate 10. Substrate 10 may be a polyimidethin-film membrane, a pad deposited on a membrane, a semiconductorwafer, a printed circuit board, or any other material requiringelectrical contacts. Usually a thin metal seed layer 11 is deposited onthe upper surface of substrate 10. Seed layer 11 may be a 100-400 Ålayer of chromium, with a 500-4000 Å layer of copper. Seed layer 11 maybe used to conduct electric current during electroplating steps.

Next, a photoresist layer 12 is deposited on the upper surface of seedlayer 11, as in FIG. 1b. Photoresist layer 12 is exposed in certainregions and then dissolved away leaving bump areas 14 a, 14 b, and 14 c.The surface of seed layer 11 in each bump area 14 is then cleaned withagents suitable for the substrate material. If substrate 10 is a contactformed on a polyimide film, its exposed areas may be cleaned by firstplasma cleaning the surface, second using an acid cleaner combined witha wetting agent, and third immersing substrate 10 in an acid dip. Thecleaned and exposed bump areas 14 are then ready for bump and spikedeposition.

Using the apparatus of FIG. 2, substrate 10 (with masking photoresistlayer 12) is placed in a tank 16 containing an electroplating solution18. One effective solution uses a concentration of 150 to 400 gramsnickel chloride to each liter of deionized water, mixed with 30 to 45grams of boric acid. Substrate 10 is connected to a current source 20,whose other terminal connects, through current regulator 22, toelectrode 24 emersed in electroplating solution 18, forming electricalcircuit 25.

Referring to FIG. 1d, a first solid layer (or base) 26 a, 26 b, 26 c foreach bump is deposited within each bump area 14 a, 14 b, and 14 c. Thesefirst layers 26, e.g., of nickel, may be deposited through a steadyapplication of current through circuit 25. Particle-less spikes 28 arethen deposited on the top surface of each of these first layers 26 bypulsing current through circuit 25. During spike deposition, theelectroplating solution 18 may be heated to approximately 55 degreesCentigrade±5 degrees. Current density may be between about 35 amps persquare foot and about 75 amps per square foot. The electroplatingsolution 18 may have a Ph between 1.1 and 3.0, and may be agitated usingpercolated N₂ air.

The current may be pulsed with a repeated duty cycle as shown in FIG. 3,where period P is broken into two sub-periods: a first, ON period 30where current I through circuit 25 is high, and a second, OFF period 32,where current I is zero. Good results have been achieved using a dutycycle period of between 0.5 seconds and 0.9 seconds, where ON period 30lasts between about 0.4 seconds and 0.8 second, and OFF period 32 lastabout 0.1 seconds. The current may be pulsed typically between 0 and 3minutes. The periodic pulsing of current through circuit 25 causesnickel spike formations 28 to grow on the exposed surfaces of each bump26. These pointed spikes 28 are grown without use of any deposited hardparticles.

Through experimentation, good growth parameters include a 55 degree C.temperature for electroplating solution 18, a current of 35 amps persquare foot, and a current duty cycle having an ON period of about 0.6seconds, and an OFF period of about 0.1 seconds. The current is pulsedfor a total of about 3 minutes.

Referring to FIG. 1e, spikes 28 deposited across the exposed surfaces ofbumps 26 have a layer of gold 34 deposited over spikes 28, to ensuregood electrical contact (FIG. 1f). Depending upon application, goldlayer 34 may not be required. Then photoresist layer 12 is stripped,leaving a series of exposed bumps 26 on the surface of substrate 10,each bump 26 having sharp spikes 28, coated with gold layer 34, as shownin FIG. 1g, and in greater detail in FIG. 4. The surface of substrate10, having such spikes 28, may be referred to as roughened.

Referring to FIG. 5, each spike may be generally characterized by itsheight H, width W, and angle from normal θ. Measurements of spikescreated by the described method had heights varying from about 0.353microns to about 1.266 microns (mean: 0.718 microns), widths varyingfrom about 0.345 microns to about 1.250 microns (mean: 0.715 microns),and an angle θ from about 10 degrees to about 45 degrees (mean: 28degrees). Photomicrographs of a contact bump 26 having deposited spikes28 are shown in FIGS. 6a (elevated) and 6 b (enlarged). The spikes arerelatively uniformly distributed across the surface of the contact witha density of 1 to 2 spikes/μm². In addition to the spikes, the surfaceof contact bump 26 exhibits rolling hills and valleys known asasperities, which do not affect the use of the electrical contact.

Each of the deposition parameters may be varied and still generatespikes on the substrate surface. Referring to FIGS. 7a through 7 f,contour plots are shown for trading off different deposition parameters.The experimental results shown in contour plot 700 a of FIG. 7a employeda duty cycle of 0.4 seconds ON, 0.1 seconds OFF, a deposition time of 2minutes, and both current density I and temperature of electroplatingsolution were varied. Contour lines 710 represent an equivalent endresult in terms of spike production, with higher numbers representingqualitatively better spikes (that is, −2 is better than −16). Contourplot 700 b (FIG. 7b) performed the same trade-offs, but with a dutycycle of 0.6 seconds ON, 0.1 seconds OFF and a deposition time of 3minutes. Contour plot 700 c (FIG. 7c) performed the same trade-offs, butwith a duty cycle of 0.8 seconds ON, 0.1 seconds OFF and a depositiontime of 3 minutes.

Likewise, contour plots 700 d through 700 f varied both current densityand plating time. Contour plot 700 d used a duty cycle of 0.8 secondsON, 0.1 seconds OFF and a temperature of 37.5 degrees C. Contour plot700 e used a duty cycle of 0.8 seconds ON, 0.1 seconds OFF and atemperature of 55 degrees C. Contour plot 700 f used a duty cycle of 0.4seconds ON, 0.1 seconds OFF and a temperature of 55 degrees C.

Again, contour lines 710 represent an equivalent end result in terms ofspike production, with higher numbers representing qualitatively betterspikes. For example, for a duty cycle of 0.8 seconds ON and 0.1 secondsoff, better results are achieved with a temperature of 55 degreescompared with 37.5 degrees (compare FIG. 7d with FIG. 7e).

Referring to FIGS. 8a and 8 b, a flexible membrane 40 (e.g., ofpolyimide film) has a contact pad 10. The surface of contact pad 10 hasbumps 26 having spikes 28, for making excellent electrical contact witha corresponding device pad on a semiconductor device.

Other embodiments are within the scope of the claims. For example,different: metal solutions may be used for electroplating. Also, thespikes and other features may be formed on a variety of differentsubstrates. Further, different time periods, current strengths, dutycycles, temperatures, and other deposition parameters may be employed.

What is claimed is:
 1. A roughened contact comprising: a contact having a surface; and a solid metal layer deposited on the surface of the contact, the solid metal layer having spikes protruding away from the contact, the spikes being substantially conical, a side of each of the spikes having an angle from normal of between 10 degrees and about 45 degrees.
 2. A roughened contact comprising: a contact having a surface; and a solid metal layer deposited on the surface of the contact, the solid metal layer having spikes protruding away from the contact, the spikes being substantially conical, each of the spikes having a base between about 0.345 microns and about 1.250 microns, a side of each of the spikes having an angle from normal of between 10 degrees and about 45 degrees.
 3. A roughened contact comprising: a contact having a surface; and a solid metal layer deposited on the surface of the contact, the solid metal layer having spikes protruding away from the contact, the spikes being substantially conical, a side of each of the spikes having an angle from normal of between 10 degrees and about 45 degrees.
 4. A roughened contact comprising: a contact having a surface; and a solid metal layer deposited on the surface of the contact, the solid metal layer having spikes protruding away from the contact, the spikes being substantially conical, a side of each of the spikes having an angle from normal of between 10 degrees, and about 45 degrees and the spikes having a density of between about 1 and about 2 spikes per square micrometer.
 5. The contact of claim 1, 2, 3, or 4, wherein the solid metal layer comprises nickel.
 6. The contact of claim 1, 2, 3, or 4 further comprising a conductive plating deposited on the surface of the solid metal layer.
 7. The contact of claim 1, 2, 3, or 4, wherein the conductive plating comprises gold.
 8. The contact of claim 1, 2, 3, or 4, wherein the spikes have a height between about 0.350 microns and about 1.275 microns.
 9. A roughened contact comprising: a contact having a surface; a solid metal layer deposited on the surface of the contact, wherein the solid metal layer comprises nickel and has spikes protruding away from the contact, wherein the spikes are substantially conical, the spikes have a height between about 0.350 microns and about 1.275 microns, the spikes have a base between about 0.345 microns and about 1.250 microns, wherein a side of each of the spikes has an angle from normal of between about 10 degrees and about 45 degrees, and wherein the spikes have a density of between about 1 and about 2 spikes per square micrometer; and a conductive plating deposited on the surface of the solid metal surface, wherein the conductive plating comprises gold.
 10. A membrane for use in testing a circuit comprising: a flexible membrane substrate; a contact on the flexible membrane substrate having a surface; and a solid metal layer deposited on the surface of the contact, the solid metal layer having spikes protruding away from the contact, the spikes being substantially conical, a side of each of the spikes having an angle from normal of between 10 degrees and about 45 degrees, and the spikes having a base between about 0.345 microns and about 1.250 microns.
 11. A membrane for use in testing a circuit comprising: a flexible membrane substrate; a contact on the flexible membrane substrate having a surface; and a solid metal layer deposited on the surface of the contact, the solid metal layer having spikes protruding away from the contact, the spikes being substantially conical, and a side of each of the spikes having an angle from normal of between 10 degrees and about 45 degrees.
 12. A membrane for use in testing a circuit comprising: a flexible membrane substrate; a contact on the flexible membrane substrate having a surface; and a solid metal layer deposited on the surface of the contact, the solid metal layer having spikes protruding away from the contact, the spikes being substantially conical, a side of each of the spikes having an angle from normal of between 10 degrees and about 45 degrees, and the spikes having a density of between about 1 and about 2 spikes per square micrometer.
 13. The membrane of claim 10, 11, or 12, wherein the spikes are substantially conical.
 14. The membrane of claim 10, 11, or 12, wherein the solid metal layer comprises nickel.
 15. The membrane of claim 10, 11, or 12 further comprising a conductive plating deposited on the surface of the solid metal layer.
 16. The membrane of claim 10, 11, or 12, wherein the conductive plating comprises gold.
 17. The membrane of claim 10, 11, or 12, wherein the flexible membrane substrate comprises a polyimide film. 